Many of the ores mined to recover valuable metals such as nickel, cobalt, copper, uranium, gold, or others also contain arsenic in appreciable quantities. The processing of these arsenic containing ores has long been recognized as a problem in the mining industry. For example, much of the contained arsenic may volatilize during pyrometallurgical treatment, such as roasting, thus producing environmentally unacceptable arsenious oxide, As.sub.2 O.sub.3. Alternatively, the arsenic may be dissolved during hydrometallurgical processing of the ores and must be removed from the resulting solution in order to produce the metal values of commercially acceptable purity. This arsenic removal can be accomplished by several different methods, including precipitation and solvent extraction. The resulting arsenic containing intermediates, e.g. precipitates, are enriched in arsenic and have to be appropriately treated to produce compounds that can be safely disposed of in existing disposal areas.
For example, common impurities dissolved in a nickel containing electrolyte include arsenic and copper. During purification of nickel containing electrolyte, H.sub.2 S may be added to remove arsenic and copper from the electrolyte. The following reactions occur with the H.sub.2 S: EQU CuSO.sub.4 +H.sub.2 S.revreaction.CuS+H.sub.2 SO.sub.4 EQU 2 H.sub.3 AsO.sub.3 +3 H.sub.2 S.revreaction.As.sub.2 S.sub.3 +6 H.sub.2 O
Other metals contained in the electrolyte, may also precipitate during this operation, as does a small quantity of the nickel. The resulting arsenic and copper containing precipitate must be suitably treated to recover any of the valuable metal constituents. The contained arsenic will eventually have to be converted to a suitable product that can be utilized or disposed of in an environmentally acceptable manner. The production of stable arsenic compounds suitable for safe disposal can be very expensive.
Copper arsenate has long been used as an ingredient for preserving wood. Wood preservative compositions containing compounds of copper, chromium and of arsenic (CCA) are in widespread use. For example, the American Wood Preservers' Association gives three standard overall compositions:
______________________________________ TYPE A TYPE B TYPE C ______________________________________ Copper, expressed as wt. % CuO 18.1 19.6 18.5 Chromium, expressed as wt. % 65.5 35.3 47.5 CrO.sub.3 Arsenic, expressed as wt. % 16.4 45.1 34.0 As.sub.2 O.sub.5 ______________________________________
Commonly, compounds used to prepare the final CCA compositions are copper arsenate and chromic acid. The copper arsenate used for the production of CCA compositions must be of high purity to ensure that no undesired reactions occur during the production of CCA compositions and that no undesired contaminants could leach out of the final wood treatment product.
W. K. Hartford in U.S. Pat. No. 4,103,000 discloses the production of slurries of copper arsenate by reacting copper metal with arsenic acid in the presence of an oxidizing agent, e.g., oxygen, and an acid catalyst such as nitric acid, sulfuric acid, or hydrochloric acid. European Patent Application 244,682 disclosed production of copper arsenate by reaction of copper metal with arsenic acid and hydrogen peroxide, e.g.: EQU 3 Cu+2 H.sub.3 AsO.sub.4 +3 H.sub.2 O.sub.2 .fwdarw.Cu.sub.3 (AsO.sub.4).sub.2 +6 H.sub.2 O
U. Dienstbach in U.S. Pat. No. 4,405,569 teaches the formation of soluble sodium arsenate during oxidative leaching of a cobalt and nickel containing concentrate with sodium hydroxide at elevated temperatures and pressures. Following removal of the insoluble metal hydroxides by filtration, the sodium arsenate solution can be reacted with copper sulfate solution to precipitate crystalline copper arsenate: EQU 3 CuSO.sub.4 +2 Na.sub.3 AsO.sub.4 .fwdarw.Cu.sub.3 (AsO.sub.4).sub.2 +3 Na.sub.2 SO.sub.4
According to Magalhaes et al., in "The chemistry of formation of some secondary arsenate materials of Cu (II), Zn (II) and Pb (II)," published in Mineralogical Magazine, December 1988, Vol. 52, pp. 679-90, several copper arsenate minerals are found in nature, such as lammerite, Cu.sub.3 (AsO.sub.4).sub.2 ; olivenite, Cu.sub.2 AsO.sub.4 (OH); and clinoclase, Cu.sub.3 AsO.sub.4 (OH).sub.3. Other copper arsenates include CuHAsO.sub.4.H.sub.2 O which is used for wood preservative compositions. Different copper arsenates can be formed depending on the conditions prevailing during their precipitation, e.g. solution composition, pH, and temperature. For example, Mirza et al. showed that crystalline basic copper arsenate, Cu.sub.2 AsO.sub.4 (OH), can be precipitated at a pH of about 4 and a temperature of about 90.degree. C. EQU 2 CuSO.sub.4 +H.sub.3 AsO.sub.4 +H.sub.2 O.revreaction.Cu.sub.2 AsO.sub.4 (OH)+2 H.sub.2 SO.sub.4
Takahashi et al. in U.S. Pat. No. 4,357,261 disclose a method for copper arsenate production from a copper, arsenic, and sulfide containing refinery by-product. However, the invention teaches that copper and arsenic are extracted into solution during leaching under acidic conditions. Copper hydroxide containing copper arsenate is then precipitated from the leach solution by addition of sodium hydroxide solution to a pH of 6-7. As reported by T. Terayama in "ARSENIC RECOVERY AND HIGH PURITY ARSENIC METAL PRODUCTION FROM ARSENIC RESIDUES OF COPPER SMELTING PROCESS," published in Metallurgical Review of MMIJ, Vol. 6, No. 2, 1989, the Sumitomo Metal Mining Co. Ltd. actually practices a slightly modified process for the production of copper arsenate. Copper and arsenic containing solution obtained after caustic leaching of decopperized copper electrorefining slimes is combined with an arsenic sulfide intermediate. Copper arsenate of the approximate composition Cu.sub.3 (AsO.sub.4).sub.2 is then precipitated following further neutralization with sodium hydroxide. The resulting product has a high moisture content of about 50% or more.
It is desirable to produce a crystalline copper arsenate product having improved handling properties and a low moisture content. Copper arsenate having a low moisture content has a reduced drying requirement and may have reduced transportation costs through weight saving.
It is an object of the present invention to provide a low cost method for producing high purity crystalline, basic copper arsenate, Cu.sub.2 AsO.sub.4 (OH).
It is a further object of the invention to produce a basic copper arsenate of low moisture content for use in the wood preservation industry from impure process intermediates and/or by-products such as arsenic sulfide containing sludges.
It is a further object of the present invention to provide an economical method for the conversion of impure amorphous copper arsenates of high moisture contents to crystalline, basic copper arsenate, Cu.sub.2 AsO.sub.4 (OH), with low moisture content and improved handling characteristics.